The present invention relates to a metal wire or a metal strip having a lubricating surface layer, a method for its preparation as well as a new use of a mixture of a fatty acid and its alkali metal soap.
Drawn wire or drawn strip is generally subjected to a mechanical processing after the drawing. Such processing calls for low friction. This problem is often solved by the addition of lubricants such as mineral or synthetic oils and cutting fluids during the processing.
However it is desirable to coat the wire/strip with a lubricating surface layer already in the delivery state, i.e. immediately after drawing (and possibly annealing), in order to facilitate the handling. Some methods therefore are already in use. One such method is the coating with a wax layer where the wax is mixed with a solvent, e.g., petrol. Another method comprises coating with silicones which like-wise is carried out by means of solvents. The problem caused by these methods relates to the working environment due to the handling of solvents during coating. Another disadvantage is that the wax must be washed away before the use of the wire/strip detergents such as trichloroethylene or alkaline detergents (generally in combination with ultrasonic treatment) is required. Due to the general desire to keep away from the use of organic solvents and compounds containing chlorine, it is thus desirable for the application of a lubricating surface layer on metal wire and metal strips to be able to use a water-soluble system having minimal influence upon the environment both during the application of the layer and its possible removal before the use of the wire/strip.
A further disadvantage of a lubricating surface layer of wax is the difficulty to achieve complete covering and an even thickness of the layer.
The manufacture of coils puts high demands on lubricating in order to give a uniform pitch and diameter as well as to avoid rips and in addition to give high productivity. For that reason the manufacturer of coils often uses different types of lubricants during the winding of the spiral in spite of the wax layer. However this should be avoided because, for instance, commercial lubricants based on mineral oil often contain aggressive additives which can be difficult to wash away. During use, the remaining residues of lubricant or additives attack the material of the wire and damage the wire in different ways.
A special case within the field of coil manufacturing is the use of resistance alloys, e.g. iron-chromium-aluminium, nickel-chromium(-iron) and copper-nickel, for the manufacture of heating coils for electrical elements. In this case extra high demands are put on uniform coils, high productivity and lenient treatment of the wire surface. Moreover high demands are put on the properties of the lubricant relating to chemical compatibility with the wire material before the use at transport and storing on one hand, and during the use on the other if the lubricant fully or partly remains on the surface.
It is known from U.S. Pat. No. 3,098,294 to coat a draw piece with a sodium soap of a fatty acid as lubricant in cold drawing a small article (specifically a cartridge case) from a draw piece made of brass or mild steel. Among the soaps investigated sodium behenate was found to give the best results whereas sodium laurate and sodium oleate gave the poorest results as regards the force required in the cold drawing which force in case of the two last mentioned soaps becomes higher than that of a comparative mixture of several fatty acid soaps.